Cockling is caused by water colour paper of less than 200lb or 430g.m-2 being subjected to water washes then allowing to dry out with the colour resting in the troughs or hollows causing uneven drying and other horrors. It is prevented by stretching the paper when wet on to a wooden ply board and securing the paper with gummed strip at the edges. The paper is allowed to dry naturally over 24 hours then painted.